The production of thin sheet steel used to produce products such as automobile bodies and household appliances is most often accomplished using a two step process. The first step is to heat the raw slab to a temperature sufficient to produce pyroplasticity in the object. The second phase occurs in the hot rolling mill where a rolling process reduces the thickness of the slab. The slabs are usually heated to rolling temperature (2000.degree. F.-2500.degree. F.) in large reheat furnaces prior to processing. The thin sheet of steel is called "strip" by those skilled in the art. It is the reheat furnace portion of the entire process of steel "strip" production to which the invention relates.
The furnace is generally either of the pusher type or walking beam type. In both types of furnace, a network of water-cooled piping commonly known as a skid system is used to support the slabs during the heating cycle. The skid system is usually water-cooled to an maintain its mechanical strength at furnace temperatures. Each skid is comprised of a horizontal and longitudinally oriented, water-cooled pipe (skid pipe) that is equipped with a steel or high temperature alloy wear bar, rider, or rail affixed (welded) to the top of the skid pipe and insulation completely covering the pipe.
Riders are the part that the slab actually rest on and support the weight of the slabs. If they are degraded by the furnace environment they can cause product defects. Riders are most commonly attached to the upper surface of the water-cooled skid pipe by "partial" penetration welding, "full" penetration welding, and "cleating." For purposes of the present invention these or any other method is acceptable.
Attachment of riders also has a remarkable effect upon the cost of required periodic replacement since the existing designs are inherently difficult to replace without penalties such as severe damage caused to the base skid member when the old rider is removed. Rider replacement also requires that the essential energy saving skid pipe insulation that covers the entire circumference of the skid must be partially removed to allow access to the areas where rider replacement occurs. The described method of rider maintenance is very labor and time intensive and almost always leads to compromise that affects ultimate cost and quality of the end product. Several existing ideas attempt to alleviate this problem.
The Harding Patent, U.S. Pat. No. 5,257,928 discloses a two piece wear bar with a mechanical locking means. A magnetic sphere is used to lock the two pieces together. The drawback to this patent is that the magnet required to move the ball must be so powerful that it seriously disrupts the replacement process and often picks up the entire skid assembly. Another draw back is that the system does not have enough contact with the water cooled skid to properly dissipate the heat.
The Heuss Patent, U.S. Pat. No. 5,136,610 discloses a multi piece assembly also includes a ceramic material used as insulation between rider and the cut out housing fitting. The ceramic insulation is the weakness of this system. The insulation becomes brittle after prolonged exposure to furnace temperatures and fails. Furthermore, the insulation itself prevents heat transfer to the cooling pipe that the housing is connected to which causes the rider to fail early.
The Webster Patent, U.S. Pat. No. 4,601,659 discloses using a hollow upper wear bar cleated to the upper surface of the structural member. The disadvantage of all hollow upper parts it the reduced strength against compression. The combination of weight and heat causes these type of wear bars to compress. This causes unevenness on the surface of the strip. Consequently, when one fails all must be replaced because the new top piece is taller than its neighbors. In fact, the Webster design virtually requires that all of the top pieces in a row be changed as you cannot get to an inside top piece without removing the others on the same row.